A furnace burns gas and moves air. Neither of those should make a puddle, so water on the floor under one surprises people. In practice it is common, usually fixable, and in most Sacramento homes it comes from one of two places. One cause is a genuine safety issue, and it is covered below.
First, shut it down and look
- Turn the system off at the thermostat. Water near a furnace means water near its control board and wiring.
- Put towels down and find where the water is coming from. The bottom of the furnace cabinet, the PVC pipes, the coil box sitting on top, or a humidifier on the side each point to a different cause.
- If the furnace is in the attic, check the ceiling below. Attic installs in Natomas, Elk Grove and newer Roseville sit above drywall. A slow leak there becomes a stain, then a sag.
Leaking only when the AC is on
This is the most common version of the question, and the answer is usually that the furnace is not leaking at all. In a typical Sacramento split system, the air conditioner’s evaporator coil sits in a box directly on top of the furnace. Every cooling cycle pulls moisture out of the air, and that water collects in a drain pan under the coil and runs out through a condensate line.
When that line clogs with algae or sludge, the pan overflows and the water runs down into the furnace cabinet and onto the floor. It looks exactly like a furnace leak. The usual causes:
- Algae in the condensate line. It grows all summer. A cup of distilled vinegar poured into the line’s access port each spring prevents most of these.
- A frozen evaporator coil that is now thawing. If the coil iced up from a dirty filter or low refrigerant, it sheds far more water than the pan can take when it melts. See why your AC is frozen.
- A cracked or rusted drain pan on older coils.
- A failed condensate pump in installations where the water has to be pumped up and out rather than draining by gravity.
Our article on AC leaking water covers the cooling side in more detail.
Leaking when the heat is on
If the water appears during heating, the type of furnace matters.
High efficiency furnaces, 90 percent or above
You can recognize these by the white PVC pipes coming out of them instead of a metal flue. A condensing furnace extracts so much heat from the exhaust that water condenses out of it, often several gallons on a cold day. That water is supposed to drain through a trap and a line. Water on the floor usually means:
- A clogged condensate trap. Sediment builds up in it. On many furnaces a blocked trap also trips the pressure switch and shuts the furnace down, so a leak and a furnace that keeps shutting off often arrive together.
- A disconnected or cracked drain line, sometimes knocked loose during other work.
- A vent pipe sloped the wrong way, so condensation runs back down the PVC and leaks at a joint.
- A failed condensate pump.
These are routine repairs. The trap cleaning is part of what a proper fall tune-up should include.
Standard efficiency furnaces, around 80 percent
These have a metal flue and should not produce meaningful water. If one is leaking during heating, take it seriously. Condensation where there should be none can mean the exhaust is not venting properly, that flue gases are cooling and condensing inside the vent because of a blockage or a poorly sized flue, or in older units that the heat exchanger is compromised.
If you have an 80 percent furnace leaking water during heating, stop using it until it is checked. Poor venting is a carbon monoxide concern. Make sure your CO alarm works. If it sounds, get everyone outside and call 911. Our article on furnace smells covers the related warning signs.
Other sources worth ruling out
- A whole-house humidifier mounted on the furnace with a stuck valve or a clogged pad.
- A water heater sitting next to the furnace in the same closet. People mistake one for the other more often than you would expect.
- A wet filter. If the filter itself is soaked, the water is coming from the coil above it.
What we do
We trace the water to its source first, because a condensate clog and a venting problem look the same on the floor and are completely different jobs. On a high efficiency furnace we clear and test the trap and drain. On any gas furnace we take a carbon monoxide reading before we leave. Our diagnostic is a flat $69 and it is credited toward the repair.
What furnace repairs typically cost here is in furnace repair cost in Sacramento. For service by area, see furnace repair in Sacramento or Roseville, and our heating services page covers the rest.
Frequently asked questions
Why is my furnace leaking water when the AC is on?
Usually the leak comes from the air conditioner’s evaporator coil sitting on top of the furnace. When its condensate line clogs, the drain pan overflows and water runs down through the furnace cabinet. Clearing the line normally fixes it.
Is a furnace leaking water dangerous?
Most leaks are condensate drainage problems and are not dangerous, though water can damage the control board. The exception is a standard 80 percent furnace leaking during heating, which can point to a venting problem. Stop using it and have it checked.
Why does my high efficiency furnace leak water?
High efficiency furnaces produce water as part of normal operation and drain it through a trap and line. A clogged trap, a disconnected drain line, a failed condensate pump or a vent pipe with the wrong slope will let that water escape.
Can I unclog the furnace condensate line myself?
Often, yes. Turn the system off, find the drain line’s access port, and flush it with distilled vinegar or clear it with a wet-dry vacuum at the outside end. If the trap itself is clogged or the furnace keeps shutting off, have it serviced.
My furnace filter is wet. What does that mean?
Water is coming from above the filter, which is almost always the air conditioner coil. A clogged condensate line or a thawing frozen coil are the usual causes.
Why does my furnace leak and keep shutting off?
On high efficiency furnaces a clogged condensate trap often does both. Water backs up and leaks, and the backup also trips the pressure switch, which shuts the furnace down as a safety measure.